Right, you're new to development, so the first thing you should know is, Joomla 2.5 reached the end of life and is no longer officially supported. Whether extension/theme developers wish to carry in supporting this version of Joomla is up to them, but I highly recommend you don't start to support it for a new project.
Also, Bootstrap was not used in Joomla 2.5, therefore would not be able to make full use of the Joomla API methods to include Bootstrap features, thus making your life harder.
Even though Bootstrap was introduced in Joomla 3.x, it doesn't mean you have to use it. My personal opinion of it, is it does what it's supposed to do very well, but it's too common now and everyone seems to be using it, and everyone's site looks the same.
I'd firstly suggest having a look around at some other frameworks. A personal favorite is UIKit (developed by Yootheme). You could also use Yootheme's Master 2 Theme (Demo Package or Template)
If it's simply a responsive grid you wish to use, you may also want to consider using only the grid of your chosen framework and stripping out all other code to improve performance.
You could also take a copy of the Joomla 3.x Protostar template which utilizes Bootstrap, else I'd suggest looking at other developers templates to get an idea of the structure.
Hope this helps